Composite video signals, such as those used in typical television receivers, include video information which is intended for display on a device such as a cathode ray tube. The video information is interspersed between synchronizing pulses to synchronize scanning of the cathode ray tube in the receiver to provide a coherent display. A signal in accordance with current FCC standards has 525 horizontal scanning lines per vertical frame. The vertical frame is divided into two vertical fields, known as the odd and even fields, with each field having 262.5 horizontal lines. Thus, the horizontal deflection rate is 262.5 times the vertical deflection rate. If this relationship is maintained exactly, the signal is called interlaced, standard, or synchronous.
Numerous video signal sources such as CATV systems, video games, video players, and the like provide non-interlaced, non-standard, or asynchronous signals. Such video sources do not necessarily maintain the precise horizontal and vertical scanning rates or the scanning rate relationship between horizontal and vertical scanning required for signals transmitted in accordance with FCC standards.
A typical prior art approach to synchronizing the scanning of a cathode ray tube with synchronizing pulses contained in a composite video signal is to provide two oscillators synchronized respectively with the horizontal and vertical synchronizing pulses. While such circuitry has been used for many years with generally satisfactory results, known prior art circuitry of this type is susceptible to noise and has poor long term stability. Furthermore, in order to provide initial acquisition and retention of sync and proper operation with non-standard signals, typical prior art oscillator circuitry has a tolerance or range of frequencies within which more or less satisfactory operation can be achieved. This tolerance, however, makes the prior art oscillator circuitry susceptible to false triggering by noise and attendant improper scanning.
Due to its relatively low frequency of operation, the vertical oscillator is highly susceptible to false triggering due to various types of noise or interference. Component drift in prior art oscillator circuitry and varying signal conditions also require the use of hold controls to adjust the frequency of operation of the oscillator as components age and signal conditions vary.
A common prior art approach used to eliminate the vertical oscillator is to provide a countdown circuit which counts pulses provided at a rate related to the horizontal scanning rate. Typical circuitry of this type uses a binary counter which provides an output pulse after the proper number of pulses have been counted. The counter output pulse is provided in a timed relationship with and substituted for the received vertical synchronizing pulse. Since the vertical output pulses provided by a counter are relatively invariant, television receivers with vertical countdown circuits incorporated therein cannot readily receive and properly display non-standard signals without substantial additional circuitry to recognize such signals and modify the operation of the countdown circuit accordingly. Prior art vertical countdown circuits which incorporate such circuitry typically switch to a driven mode of operation for non-standard signals. Incorporation of such recognition circuitry and alternative mode of operation, however, results in substantial added circuitry and added expense making the use of countdown circuitry prohibitively expensive in many applications.